Very little is known about the Breen. They are thought to be humanoid, as they appear to be of typical humanoid height and build, though Ezri Dax speculated that they had a wolf-like appearance (DS9episode: "Strange Bedfellows"), and xenologists speculated that the Breen could be a silicon-based lifeform. No one has seen the Breen outside of their sealed, refrigerated suits of armor; it is said that those who have did not live to tell of it. (DS9episode: "Strange Bedfellows") Since it had been reported that the Breen homeworld was a frozen wasteland, it was thought that the refrigerated suits were intended to keep them comfortable in warmer alien climates, though Weyoun once mentioned that Breen is "actually quite comfortable" (DS9episode: "The Changing Face of Evil"), leaving the purpose of their suits a source of speculation.

It was speculated that the Breen body may be composed of ammonia and gelatin, which are held together in freezing temperatures but evaporate in warmer temperatures. Evaporation upon contact with warm temperature would account for why Gul Dukat and Major Kira failed to report on the Breen appearance after stealing the suits of two Breen guards (DS9episode: "Indiscretion"). They likely have a skeletal structure but, as they have no blood, contain few organs. The internal structure of the Breen body is said to consist of layers of gelatin-filled chambers, with the gelatin serving a similar function to blood, as it transfers chemicals vital for bodily functions. Therefore, when a Breen sustains an injury, a layer of gelatin will clot around it and seal the wound, which will leave a scar if left untreated (Decipher RPGmodule: Aliens).

The Breen are often regarded as ruthless and devoid of humanoid compassion, a depiction that is augmented by their lack of blood (DS9episode: "By Inferno's Light"). During the Dominion War, their unprecedented attack on Starfleet Headquarters was seen as a symbol of their ferocity. The Romulans have a saying: "Never turn your back on a Breen" (DS9episode: "By Inferno's Light").

It was not until 2382 that definitive information was obtained relating to the Breen and their biology. Initially it was believed that the Breen were a single race; however, as detailed by Sarina Douglas and Julian Bashir, the name Breen is a name taken on by the member species of the Breen Confederacy. It has been claimed that the individual races (the Amoniri, Fenrisal, Paclu, Silwaan) take on the singular name and the concealing uniforms in order to prevent any biases and prejudices to interfere with the development of their society. The uniform design has been designed to support the distinctive features of the individual races. (ST - Typhon Pactnovel: Zero Sum Game)

Little was known about the race as they tended to keep to themselves. What was known was that they were both secretive and aggressive, though it must be stated that they did not exalt violence for its own sake and that it was aggression that served a purpose. Furthermore, they were known to intentionally keep their motives and feelings hidden from other races which, when combined with the fact that they cannot be sensed by Betazoids, further adds to the mystery of the Breen. Common speculation among other races was that the Breen were more open to their own kind but this was never confirmed. Their spoken language (at least to non-Breen) sounds like uncomprehensive static, although this electric chatter is understood by Vorta and Founders.

Members of this race were not excessively individualistic and were in fact commonly encountered in groups. Furthermore, the level of secrecy that they have maintained about their society and homeworld hints at a high level of clan cohesion or possibly even a government structure. This form of society has yet to produce any form of rebel that was willing to break the silence about their customs, culture or homeworld. When alone, Breen tend to act unpredictably, with displays of neurosis due to the fact that they are working apart from their group. While this hinted that Breen are incapable of working alone, xenologists have stated that this behavior may in fact be faked in order to produce the illusion of such traits in order to fool other races. (Decipher RPGmodule: Aliens)

The Breen were disliked by the Romulans, who had a saying: "Never turn your back on a Breen". (DS9episode: "By Inferno's Light") The nature of why the Romulans distrust the Breen so much remains unknown. They, however, earned this reputation through their opportunistic tendencies of preying on the conflicts of other species while at the same time, rarely interacting directly. This was because they wished to profit on such activities which included slavery. (Decipher RPGmodule: Aliens, DS9episode: "Indiscretion") Ultimately, the Breen acted for their own self-interest and thus engaged in ruthless actions without caring for other species. Their motivations for doing so were always a secret and the Breen never revealed what their next actions were going to be. (Decipher RPGmodule: Worlds)

What was known about the Breen race was that they tended to act out of their own self-interest with little weight placed on the hardships by those that stand in their way. They appeared callous to the suffering of others and in fact do not heed calls for aid unless there was something to gain from such an endeavor; be it monetary, technology or simply territory. (Decipher RPGmodule: Aliens) While the Breen were quite capable of acts of deceit, Elias Vaughn thought it unlikely they would attempt to build up a military presence in hiding-as it did not fit with their cultural psychology. (DS9 - Avatarnovel: Book One) It's known that they are not above using others in their schemes where the relationship was that of a pawn rather then a partner. Regional conflicts among other empires was noted to result from Breen provocation, with the race being blamed for activities that they themselves have not committed even. (Decipher RPGmodule: Aliens) Their society also was believed to contain some aspects of a manifest destiny. (TNGnovel: The Buried Age)

Though elements of their society remain a mystery, it was known they possessed two genders and that it was typical for them to bear children at a young age. The reason for this puzzled xenologists who conclude that the Breen possibly suffer from a high mortality rate and that they encourage this behavior in order to ensure their population remains at a steady level. Another theory was that they had a shorter lifespan compared to Humans and simply reached maturity quicker. However, the truth of the matter continues to be unknown. (DS9episode: "For the Uniform", VOYepisode: "Elogium")

The Breen were politically nonaligned and did not take part in official treaties or alliances, though their representatives on occasion visit certain diplomatic gatherings. However, the selection process of which gathering they do attend was a mystery to other races. What was determined, however, was that they seek to gather information in such meetings, not to participate in them. The fact that they do not appear to be an expansionistic race has meant that other races typically leave the Breen alone.

The government itself appeared to resemble a form of primitive tribalism, except that all Breen were a member of the same tribe. Members were noted to form into groups where different individuals assume different roles on a voluntary basis rather then an autocratic dictate. Their leaders rule as long as their groups allow them to do so. Their starship tactics follow a similar role, with each vessel seeming to possess a high degree of autonomy. These ships appear to only work with others through mutual consensus or agreed-upon tactics. (Decipher RPGmodule: Aliens)

Breen refrigeration suits appear to be of military issue, as they have different designs to signify various ranks. Command officers wear golden lining on the shoulders and arms, as well as distinctive gold stripes that run down the center of the top of the helmet and down the top of the "snout" section of the helmet.

Their technology consisted of an element of biotechnology that as based on the algae on their homeworld. The Breen learnt to manipulate the algae genetic structure and use it to create biomaterials with special properties such as metal, plastic, silicon as well as other such substances. Such a key trait meant that every Breen vessel included its own hydroponics bay where damage components could be regrown. (TNGmodule: Star Trek: The Next Generation Core Game Book)

During the 22nd century, the Breen were largely confined to their own home system with few forays being outside their region of space due to refrigeration technology being too primitive to allow them to survive on other worlds. (Decipher RPGmodule: Aliens)

By the 23rd century, the race had improved their environmental suits to the point that they had the capacity to travel far greater distances. It was during this period that the race offered their services to other governments that required covert agents or mercenaries. Despite this expansion into space, the Breen generally kept to themselves, with the Federation not being aware of the existence of the species. (Decipher RPGmodule: Aliens) The Romulans made first contact with the Breen in 2268 and were soon using them in covert operations and illegal work. (Last Unicorn RPGmodule: All Our Yesterdays: The Time Travel Sourcebook)

During this time, the Breen made a reputation for themselves in both the Alpha and Beta Quadrants by working as mercenaries, pirates as well as freebooters. They were considered valuable hired help as well as fearsome brigands who were augmented by the use of cloaking technology as well as other advanced forms of weaponry. (Decipher RPGmodule: Worlds)

In the 24th century, they had fully developed the means to join the galactic community but remained inscrutable with their goals being enigmatic to other empires. They tended to enter specialized markets to leverage skills in intelligence work, mining and technology that dealt with survival on colder environments. The Federation had become fully aware of this empire's existence and recognized that they had developed several technologies like the disruptor from trade with the Romulans or other races. (TNGmovie & novelization: Generations) However, it was concluded that the mysterious insular species were neither a threat or potential ally. (Decipher RPGmodule: Aliens) In 2328, the Federation constructed Deep Space 3 to monitor the Breen and those species that resided in their space. (Last Unicorn RPGmodule: Star Trek Deep Space Nine: Core Game Book) Relations between the Federation and the Breen were described as tenuous at best by the year 2360. (TNGnovel: The Buried Age)

During Benjamin Sisko's time on the USS Saratoga, the starship encountered a hostile Breen ship near Guldammur IV. In a plan devised by Sisko and Zar, a narrow-gauge phaser spread was fired from the weapons of the Saratoga, making several small gaps in the Breen's shields. This allowed the Saratoga crew to beam unwanted items into the Breen's weapon banks, clogging them and leaving the Breen unable to fire. (DS9novel: Saratoga)

In 2366, the Breen attacked the Ravinok and forced it to crash on Dozaria. The Bajoran and Cardassian survivors were forced to mine dilithium for the Breen. (DS9episode: "Indiscretion") Following the discovery of this incident, the Cardassian government was unwilling to open a potential conflict with another power as they were already engaged in conflict with the Federation. Thus, the Cardassian Central Command registered a diplomatic protest to the Breen over the attack on the Ravinok, though they later let the matter go when the Breen chose the ignore it. (Decipher RPGmodule: Worlds)

The Breen continued to be a source of some concern following the war. In April2376 the USS Enterprise-E was sent to investigate the Badlands for a Klingon reported Breen presence. Nothing was found and Elias Vaughn thought it unlikely the Breen would build up a military presence in hiding, though it was consistent with other, need-to-know only, reports he had received not long before. (DS9novel: Avatar, Book One)

Cole of Section 31 told Julian Bashir several weeks later that he thought it likely the Breen would be watching the Federation's borders and would make raids when they found a weakness. However, Cole also told Bashir he thought the Breen could be useful in fighting off a future Borg attack. (DS9novel: Abyss)

An unusual amount of Breen activity was detected along the Federation-Cardassian border near StarbaseDeep Space 10 in mid 2377. They were briefly considered as possible culprits in a series of malfunctions that had plagued the station, but eventually ruled out. (CoEeBook: Troubleshooting)

After learning of the behind-the-scenes deal-making, the Confederacy joined with the Tholians, along with four other local powers, to form the Typhon Pact. (STnovel: A Singular Destiny)

In 2382, the Breen accepted a mission on behalf of the Pact and sent an operative to Utopia Planitia Fleet Yards to capture data pertaining to the quantum slipstream drive. The Breen were soon after contracted to build the first Typhon Pact prototype slipstream vessel. The lead researcher of the project, Thot Keer, intentionally withheld information from the other members relating to the drive's development in order to promote the Breen to a level of authority within the Pact as equals to the Romulans. Shortly before the initial trials, however, the prototype, its designer, and all information relating to the project was destroyed. (ST - Typhon Pactnovel: Zero Sum Game)